1. Field of the Invention
The present invention relates to a wire material positioning connector, particularly to an optical connector that uses guide pins to couple optical connector ferrules together for fixing optical fibers in position in lightwave communications systems. The present invention also relates to a process for procuding such a connector.
2. Prior Art
FIG. 1 is a sketch of a typical example of the prior art optical connector. In FIG. 1, the numeral 10 denotes an optical connector ferrule formed by resin molding; a ribbon fiber (A) contains five optical fibers 15 which are fixed in position side by side at a pitch of, say, 0.3 mm, and guide pin holes 16 are formed on both sides of the ribbon fiber at a pitch of 3.6 mm. Indicated by 17 is a guide pin having a diameter of, say, 0.7 mm. Two such guide pins 17 are inserted into guide pin holes 16 formed in one optical connector ferrule 10, then inserted into the corresponding holes in the other ferrule which is positioned in registry with the first ferrule, and the two optical connectors are coupled together.
In the present invention, the term "optical connector ferrule" is used to designate the most important basic member of an optical connector which fixes optical fibers in position so as to couple them together.
When coupling multi-fiber optical connectors 10, it is necessary that the optical fibers in one connector should abut in a very precise manner against those in the other connector in order to minimize the coupling loss. This requirement is particularly stringent when coupling single-mode fibers which have a core diameter of only about 10 .mu.m and even an axial displacement of no more than about 1 .mu.m between fibers will cause a great coupling loss. To avoid this problem optical fibers to be mounted on an optical connector must be positioned in the proper place with very high precision. In coupling two optical connectors, it is also necessary that they should be positioned with high precision by means of guide pins.
In order to realize low-loss coupling of multi-fiber optical connectors of the type described above, it is first of all necessary that optical fiber guide holes and guide pin holes should be provided at designated positions by precision machining. But even if this requirement is met, some clearance still remains both between the optical fiber guide holes and optical fibers and between the guide pin holes and guide pins and because of the existence of such clearance, it is not always possible to achieve low coupling loss. Even if the clearance is only about 0.5 .mu.m both between optical fiber guide holes and optical fibers and between guide pin holes and guide pins, optical fibers will experience an axial offset of about 1 .mu.m in an extreme situation.
Therefore, in order to realize low coupling loss, it is essential that each of these clearances be entirely eliminated or reduced as close as possible to zero. However, in consideration of the variation in the inside diameter of optical fiber guide holes and guide pin holes, as well as the variation in the outside diameters of optical fibers and guide pins, it is by no means easy to reduce these clearances. If an optical fiber guide hole should have an inside diamete smaller than the outside diameter of an optical fiber to be inserted thereinto, not only is it impossible to insert the optical fiber but also the fiber itself may break. If the inside diameter of a guide pin hole is smaller than the outside diameter of a guide pin to be inserted thereinto, the guide pin, which must be forced into the guide pin hole, will break either itself or the guide pin hole.
Suppose that guide pins are to be inserted into guide pin holes in a multi-fiber optical connector. If the precision of machining is within .+-.1 .mu.m both for the guide pin diameter which should be 0.700 mm and for the inside diameter of the guide pin hole which should be 0.701 mm, there is a high likelihood that some of the guide pins will have an outside diameter of 0.701 mm while some guide pin holes have an inside diameter of 0.700 mm. In this situation, none of the guide pins can be inserted into any guide pin holes.
As described above, the efforts so far made to minimize the clearance that exists between guide holes and wire materials when the latter are to be fixed in position in the former have been limited by the precision of machining and this means that there is a certain limit on the effort toward reducing the coupling loss that occurs in coupling connectors such as multi-fiber optical connectors. Therefore, these problems have been a major obstacle to the goal of achieving low-loss coupling of optical connectors.
With an optical connector of the type shown in FIG. 1, some clearance (g), as shown in FIG. 2, is necessary for permitting a guide pin 17 to be smoothly inserted into a guide pin hole 16 in an optical connector ferrule 10. However, the provision of this clearance (g) causes the problem that the coupling loss occurring in coupling optical connectors, in particular, those optical connectors which are used to achieve precise positioning of single-mode fibers with a core diameter of 10 .mu.m, will vary as a result of repeated connect/disconnect cycles.
Stated more specifically, if a variation of 0.5 .mu.m occurs on account of the clearance (g) as a result of repeated connect/disconnect cycles, the optical connectors that were initially connected to produce a coupling loss of 0.5 dB will eventually produce a variation of about .+-.0.3 dB after repeated connect and disconnect operations. This amount of variation is greater than the initial value of coupling loss. It is therefore clair that in order to realize low-loss and stable coupling of single-mode fibers, the connect/disconnect variation resulting from the clearance (g) for guide pins 17 must be reduced. However, if the clearance (g) is reduced, it becomes difficult to smoothly insert the guide pin 17 into the guide pin hole 16.
These problems could be solved by employing a guide pin that is in the form of an elastic slit pipe 18 which has a longitudinal slit 18a as shown in FIG. 3. However, in order to fabricate a slit pipe having an outside diameter of 0.7 mm with a wall thickness of 0.1 mm, the pipe must be worked to have an inside diameter of 0.5 mm and it is difficult and very expensive to achieve by such micro-machining the precise working of the pipe to attain perfect roundness in its outside diameter and accuracy in other shape parameters.
In order to reduce the size of an optical connector, the size of guide pins must also be reduced but to this end, the diameter of each guide pin has to be decreased to 0.5 mm and even to 0.3 mm, making the machining of an elastic slit pipe 18 more and more difficult.
FIG. 4 is a cross section o another example of the multi-fiber silicon chip array optical connector 160 which is conventionally used as an optical fiber connecting member. Two silicon chip guides 161 that are etched on both surfaces are stacked on each other, with optical fibers (A) being aligned on the mating surfaces. A silicon guide plate 162 having coupling guide grooves is fitted onto the other surface of each silicon chip guide 161 and the assembly is fixed with a clip plate (not shown) to secure the coupling of the fibers.
The above-described problems are absent from the optical connector shown in FIG. 4 since it does not employ any guide pins. However, in this type of optical connector, fiber coupling is achieved using the guide grooves and the coupled fibers must be subsequently clamped with a clip plate. Therefore, this arrary connector cannot be connected or disconnected or switched to another channel as readily as in the case of ordinary optical connectors. Furthermore, this array connector, which is made of silicon, is vulnerable to impact and may be easily nicked at side edges upon application of impact such as the one produced when it is dropped. In addition, even if the coupled fibers are fixed with a clip plate, the compressive force is not directly exerted upon the faces at which the fibers are coupled together, so the joint provided by this type of optical connector is comparatively weak to a tensile force.
FIG. 5 is a cross section showing a further example of the prior att optical fiber coupling member in which a grooved substrate having optical fiber guide grooves formed thereon is joined to a plate with a layer of an adhesive material interposed therebetween. As shown, a plate 201 having a layer of an adhesive material 203 provided on she underside is stacked on a grooved substrate 202 having optical fiber guide grooves 204 and guide pin grooves 205 formed in its top surface, and the plate 201 and the grooved substrate 202 are joined with the layer of an adhesive material 203 to form optical fiber guide holes and guide pin holes.
As is clear from FIG. 6 which shows the interface between the plate 201 and the grooved substrate 202 on an enlarged scale, the layer of an adhesive material 203 is present not only at the interface between the plate 201 and the substrate 202 but also on the upper plate 201 over an optical fiber guide groove 204, and the thickness of the layer 203, t.sub.3, present in the latter area is generally greater than the thickness of the layer 203, t.sub.1, present at the interface between the plate 201 and the substrate 202 by an amount that corresponds to the excess portion of the adhesive material which has been displaced from that interfacial area.
Each of the optical fibers (A) inserted into optical fiber guide holes are supposed to be held in position by establishing contact with three points (a) (see FIG. 7) in each optical fiber guide groove 204. Even if a layer of an adhesive material 203 is present between the plate 201 and the grooved substrate 202 as shown in FIG. 8, an optical fiber (A) having the same diameter as that of the fiber shown in FIG. 7 is held in position by establishing contact with three points in an optical fiber guide groove 204 if the adhesive layer 203 has a uniform thickness as in the case shown in FIG. 8.
However, in fabricating the optical fiber coupling member shown in FIG. 6, the plate 201 is joined to the grooved substrate 202 with some pressure being applied to cure the layer of an adhesive material 203, and this causes the adhesive layer 203 present between the two members to be displaced toward an optical fiber guide groove 204. As a result, the thickness of the adhesive layer 203, t.sub.3, present over the groove 204 will become greater than its thickness at the other areas, t.sub.1, as shown in FIG. 6, thereby making it impossible for an optical fiber with a predetermined outside diameter to be inserted into the groove 204.
The adhesive layer over optical fiber guide groove 204 has the additional disadvantage that it easily picks up dust particles and other foreign matter by absorption or adsorption and produces high and low spots in the wall of grooves 204. The deposition of such dust particles often makes it difficult for optical fibers to be smoothly inserted into grooves 204. In addition, dust particles can cause inaccuracy in the measurement of the cross-sectional profile of grooves 204 by producing a blurred hole contour.
FIGS. 9A and 9B illustrate a still further example of the prior art optical fiber coupling member. As shown, optical fibers (B) are inserted into optical fiber guide grooves 323 that have bee formed in the top surface of a substrate 321, and a holding plate 322 is placed on the grooved substrate 321 with a layer of an adhesive agent being interposed, and the combination is pressed from above to have the optical fibers (B) fixed in the optical fiber guide grooves 323.
However, this prior art optical fiber coupling member has the following problems.
(1) The coupling member is assembled by clamping optical fibers in such a manner that they are sandwiched between the grooved substrate and the holding plate. This method is not suitable for assembling a multi-fiber coupling member since the need to employ the grooved substrate and the holding plate makes the handling of optical fibers complicated and presents much difficulty in achieving precise assembly operations.
(2) An adhesive agent applied between the grooved substrate and the holding plate might flow out from the laterial side of the gap between the two components and it is by no means simple to deal with the outflow of the adhesive agent.
(3) If an adhesive agent is applied to the optical fiber guide grooves before optical fibers are set in these grooves, it is not easy for the operator to be sure that he is installing the fibers in the right place. If an adhesive agent is applied after optical fibers have been set in the guide grooves, the lower part of the guide grooves will not be entirely filled with the adhesive agent.
As shown in FIGS. 10A to 10C, an optical connector ferrule 20 formed by resin molding has two guide pin holes 421 formed therein. Individual optical fibers (B) in a fiber array (A) are precisely fixed in position with respect to the guide pin holes 421. The two guide pins 422 are first inserted into the guide pin holes 421 in one connector ferrule, then inserted into the corresponding holes in the other ferrule which is positioned in registry with the first ferrule, thereby allowing the two ferrules to be coupled together as shown in FIG. 10C. The optical connector shown in FIG. 10 is of the four-fiber type employing guide pins having an outside diameter of 0.7 mm; the individual optical fibers (B) are arranged at a pitch of 0.25 mm on the line connecting the axes of guide pin holes 421 which are spaced apart by a distance of 3.600 mm.
For practical applications, the optical connector ferrule described above are subjected to appropriate secondary working depending upon its specific use. For instance, the ferrule is flanged or, as shown in FIG. 11A, is accommodated in a housing to form an optical connector plug 23 that can be easily connected or disconnected from an optical connector adapter 424. Alternatively, in order to secure the coupling of two ferrules, they are fixed with a clip 425 of the type shown in FIG. 11A.
The prior art optical connector shown in FIGS. 10A to l0C which uses guide pins to achieve coupling has the problem that there is no way of knowing which of the two ferrules 420 will retain guide pins 422 after disconnecting the optical connectors. If, in the coupling method shown in FIG. 11A which employs two optical connector plugs 423 and an adapter 424, the plug into which the guide pins 422 were initially inserted is disconnected, it sometimes occurs that the guide pins are left behind in the other plug. If the other plug is situated in a machine or a casing, or if it is a component on the wall such as a receptacle or an optical outlet, the guide pins on another optical connector cannot be coupled to the other plug in the adapter unless the operator is sure that no guide pin is left in the other plug.
This randomness of the site in which guide pin are left behind after disconnecting optical connectors could be dealt with by permanently fixing guide pins in selected optical connector ferrules with an adhesive agent, but then it becomes impossible to couple two connector ferrules if each of them has guide pins permanently fixed therein. Another approach would be that one guide pin is permanently fixed in every connector ferrule on a predetermined side, for example, on the right-hand side of the ferrule. But the applicability of this method is limited since in lightwave communications it sometimes becomes necessary to couple two ribbons of optical fibers with matching being established between the identification numbers of the individual fibers and, in such a case, one ribbon fiber may be required to be turned by 180.degree. before it is coupled to the other ribbon fiber.
Another problem involved in permanently fixing guide pins in optical connector ferrules is that deformed guide pins cannot be replaced by normal pins and that the end faces of connector ferrules at which they are coupled together cannot be reground and repolished as required. For these disadvantages, the idea of permanently fixing guide pins in optical connector ferrules has not gained popularity in commercial operations